A conventional vibrating type transducer for measuring the density of a fluid on the basis of the resonant frequency of a vibrator is shown in FIG. 1 as it is disclosed in U.S. Pat. No. 3,677,067. In FIG. 1, a measuring tube 1 is arranged within a fluid channel in which a fluid being monitored flows in the direction of G. The measuring tube 1 is normally arranged that its axial direction conforms to a direction of flow. A vibrator, in the form of a rectangular plate having edges on both sides fixed to the inner surface of the measuring tube 1 to include its central axis is fixed to one end of a cylindrical body 5 whose central axis is perpendicular to the surface of the vibrator 4. A cylindrical connecting case 6 is fixed to the other end of the body 5. The cylindrical body 5 is provided with a threaded portion 3 for fitting the measuring tube 1 into the fluid channel in the manner above described. The vibrator 4 is made to promote self-excited flexural vibration resulting from arcuate bending of the edges not fixed to the measuring tube 1 by means of a mechanism (not shown), so that the vibrator 4 vibrates at the resonant frequency (Fn) of the vibrating system including the vibrator 4. When the measuring tube 1 is arranged in the fluid channel, the fluid being monitored is passed through the measuring tube 1 and the fluid in contact with the vibrator 4 is also caused to vibrate as the vibrator 4 experiences flexural vibration. The mass of the vibrating system including the vibrator 4 increases to the extent of the mass of the fluid being monitored when the fluid vibrates because of the vibrator 4. As a consequence, the value of the above resonant frequency (Fn) is different from that when the fluid is not in contact with the vibrator 4. The resonant frequency (Fn) is expressed by Eq. (1). ##EQU1## where M=mass of vibrator 4; K=spring constant; and M1=mass of fluid as the above additional mass.
K, M in Eq. (1) are constants independent of the properties of the fluid, whereas Ml corresponds to the density of the fluid. As is apparent from Eq. (1), the density of the fluid can be measured by monitoring the frequency of vibration (Fn). The device shown in FIG. 1 is arranged so that the density can thus be measured and the relation of the density .rho. of the fluid to Fn conceptually becomes what is shown in FIG. 2.
In FIG. 2, Fno represents a value of Fn when .rho.=0, i.e., when the vibrator 4 is within a vacuum. When the fluid being monitored is a qas, the mass Ml of the qas vibrating as the vibrator 4 vibrates is significantly smaller than that of the vibrator 4. Because a gas has compressive properties and a low density .rho. the slope of the plot of Fn versus .rho. is very small. It is consequently difficult to measure the density of gas by means of the density measuring instrument shown in FIG. 1 because the frequency change corresponding to the density change is insufficient. Another problem is that the temperature range applicable to the density measuring instrument is narrow because the frequency (Fn) fluctuates as a result of changes in the dimensions of the vibrator 4. When the ambient temperature changes become greater than the change of the frequency (Fn) corresponding to the change of the density of the gas this effect cannot be disregarded, provided that the ambient temperature changes sharply
The present invention is intended to solve the above problems inherent in such conventional devices, specifically it is therefore an object of the invention to provide a vibrating type transducer capable of accurately checking a low density fluid such as gas having compressive properties over a wide range of measuring temperatures.